Vapor Pressures ofHe3—He4Mixtures Below the Lambda-Point

A series of measurements of the vapor pressures of solutions of ${\mathrm{He}}^3$ in ${\mathrm{He}}^4$ are described. The methods used are absolute, giving directly the vapor pressure as a function of temperature, liquid phase concentration, and vapor phase concentration. The concentrations are determined from the measured abundance ratios of the room temperature gases without the necessity of withdrawing samples from the cryostat. The ${\mathrm{He}}^3$ concentrations studied ranged up to 80 percent for the vapor phase and 13 percent for the liquid phase, while the temperature interval was 1.30-2.18°K. From these measured pressures are deduced the vapor pressures as a function of temperature and vapor phase concentration and as a function of temperature and liquid concentration, the ${\mathrm{He}}^3$ distribution function, and latent heats of evaporation. Analysis of the experimental results on rather general grounds shows there is a significant repulsive potential energy between the two isotopes. At constant temperature the mixing should thus be accompanied by heat absorption, while adiabatic mixing should lower the temperature. On the basis of classical solution theory, the mutual repulsive potential energy leads to a separation into two liquid phases around 0.7°K.